薄膜资料(Thin film Material)连续送第4部分!

xx_912 · 发表于 2006-5-5 19:55:24

1.2 Film deposition methods
Physical vapor deposition (PVD) and chemical vapor deposition (CVD) are
the most common methods for transferring material atom by atom from
one or more sources to the growth surface of a ¯lm being deposited onto
a substrate. Vapor deposition describes any process in which a solid immersed
in a vapor becomes larger in mass due to transference of material
from the vapor onto the solid surface. The deposition is normally carried
out in a vacuum chamber to enable control of the vapor composition. If the
vapor is created by physical means without a chemical reaction, the process
is classi¯ed as PVD; if the material deposited is the product of a chemical
reaction, the process is classi¯ed as CVD. Many variations of these basic
vapor deposition methods have been developed in e®orts to balance advantages
and disadvantages of various strategies based on the requirements of
¯lm purity, structural quality, the rate of growth, temperature constraints
1.2 Film deposition methods 7
source
base
pressure, Pb
h
substrate
Ts
θ
vacuum
Fig. 1.2. Schematic showing the basic features of evaporative deposition system.
and other factors. In this section, the salient features of these processing
methods are brie°y described. This is of general interest because the state
of stress in a ¯lm can be strongly in°uenced by its deposition history, as
described in the later sections of this chapter.
1.2.1 Physical vapor deposition
Physical vapor deposition is a technique whereby physical processes, such
as evaporation, sublimation or ionic impingement on a target, facilitate the
transfer of atoms from a solid or molten source onto a substrate. Evaporation
and sputtering are the two most widely used PVD methods for depositing
¯lms.
Figure 1.2 schematically illustrates the basic features of evaporative
deposition. In this process, thermal energy is supplied to a source from which
atoms are evaporated for deposition onto a substrate. The vapor source con-
¯guration is intended to concentrate heat near the source material and to
avoid heeding the surroundings. Heating of the source material can be accomplished
by any of several methods. The simplest is resistance heating of
a wire or stripe of refractory metal to which the material to be evaporated
is attached. Larger volumes of source material can be heated in crucibles
of refractory metals, oxides or carbon by resistance heating, high frequency
induction heating, or electron beam evaporation. The evaporated atoms
travel through reduced background pressure p in the evaporation chamber
and condense on the growth surface. The deposition rate _R of the ¯lm is
commonly denoted by the number of atoms arriving at the substrate per
8 Introduction and Overview
unit area of the substrate per unit time, by the time required to deposit
a full atomic layer of ¯lm material, or by the average normal speed of the
growth surface of the ¯lm. The deposition rate or °ux is a function of the
travel distance from the source to the substrate, the angle of impingement
onto the substrate surface, the substrate temperature Ts, and the base pressure
p. If the source material (such as Cr, Fe, Mo, Si and Ti) undergoes
sublimation, su±ciently large vapor pressures may be obtained below its
melting temperature so that a solid source could be employed for evaporative
deposition. On the other hand, for most metals in which a su±ciently
large vapor pressure (» 10−3 torr, or 0.13 Pa) cannot be achieved at or
below the melting temperature, the source is heated to a liquid state so as
to achieve proper deposition conditions.
Metal alloys, such as Al{Cu, Co{Cr or Ni{Cr, can generally be evaporated
directly from a single heated source. If two constituents of the alloy
evaporate at di®erent rates causing the composition to change in the melt,
two di®erent sources held at di®erent temperatures may be employed to
ensure uniform deposition. Unlike metals and alloys, inorganic compounds
evaporate in such a way that the vapor composition is usually di®erent from
that of the source. The resulting molecular structure causes the ¯lm stoichiometry
to be di®erent from that of the source. High purity ¯lms of
virtually all materials can be deposited in vacuum by means of electron
beam evaporation.
Molecular beam epitaxy (MBE) is an example of an evaporative method.
This growth technique can provide ¯lm materials of extraordinarily good
quality which are ideal for research purposes. However, the rate of growth
is very low compared to other methods, which makes it of limited use for
production of devices. In MBE, the deposition of a thin ¯lm can be accurately
controlled at the atomic level in an ultra-high vacuum (10−10 torr, or
1.33£10−8 Pa). A substrate wafer is placed in the ultra-high vacuum chamber.
It is sputtered brie°y with a low energy ion beam to remove surface
contamination. This step is followed by a high temperature anneal to relax
any damage done to the growth surface during preparation. The substrate
is then cooled to the growth temperature, typically between 400 and 700 ◦C,
and growth commences by directing atomic beams of the ¯lm material, as
well as a beam of dopant material if necessary, toward the growth surface of
the substrate. The beams are emitted from crucibles of the growth materials
which have been heated to temperatures well above the substrate temperature
to induce evaporation and condensation. The ¯lms may be examined
by transmission electron microscopy or x-ray di®raction after cooling. The
complete history of evolution of internal stress in the ¯lm during deposi1.2
Film deposition methods 9
tion can be obtained in situ by monitoring the changes in curvature of the
substrate on which the ¯lm is deposited as described in detail in Chapter 2.
vacuum Ar
Sput t er
gas
t arget
(cathode)
subst rat e
(anode)
glow discharge (Ar+)
DC
volt age
source (or)
Fig. 1.3. Schematic showing the basic features of a dc sputter deposition system.
In sputter deposition, ions of a sputtering gas, typically Ar, are accelerated
toward the target at high speed by an imposed electric ¯eld. The initial
concentration of charge carriers in the system is signi¯cantly increased
with an increase in the dc voltage, as the ions collide with the cathode,
thereby releasing secondary electrons, and with the neutral gas atoms. As
critical numbers of electrons and ions are created through such avalanches,
the gas begins to glow and the discharge becomes self-sustaining. Gaseous
ions striking the target or the source material from which the ¯lm is made
dislodge surface atoms which form the vapor in the chamber. The target is
referred to as the cathode since it is connected to the negative side of the
direct current power supply. Figure 1.3 schematically shows the basic elements
of a sputter deposition system. The chamber is evacuated and then
Ar gas, at a pressure of approximately 13.3 Pa (10−1 torr), is introduced for
the purpose of maintaining a visible glow discharge. The Ar+ ions bombard
the target or cathode, and the ensuing momentum transfer causes the neu10
Introduction and Overview
tral atoms of the target source to be dislodged. These atoms transit through
the discharge and condense onto the substrate, thus providing ¯lm growth.
Several di®erent sputtering methods are widely used for the deposition
of thin ¯lms in di®erent practical applications: (i) dc sputtering (also
commonly referred to as cathodic or diode sputtering), (ii) radio frequency
(rf) sputtering with frequencies typically in the 5{30 MHz range, (iii) magnetron
sputtering, where a magnetic ¯eld is applied in superposition with a
parallel or perpendicularly oriented electric ¯eld between the substrate and
the target source, and (iv) bias sputtering, where either a negative dc or rf
bias voltage is applied to the substrate so as to vary the energy and °ux of
the incident charged species.
There are many distinctions between the sputtering process and the
evaporative process for ¯lm deposition, as described by Ohring (1992) for
example. Evaporation is a thermal process where the atoms of the material
to be deposited arrive at the growth surface with a low kinetic energy. In
sputtering, on the other hand, the bombardment of the target source by Ar+
ions imparts a high kinetic energy to the expelled source atoms. Although
sputter deposition promotes high surface di®usivity of arriving atoms, it
also leads to greater defect nucleation and damage at the deposition surface
because of the high energy of the atoms. While evaporation occurs in a
high vacuum (10−6 to 10−10 torr, or 1.33£10−4 to 1.33£10−8 Pa), sputtered
atoms transit through a high pressure discharge zone with a pressure
of approximately 0.1 torr (13.33 Pa). Sputter-deposited ¯lms generally contain
a higher concentration of impurity atoms than do ¯lms deposited by
evaporation, and are prone to contamination by the sputtering gas. As a
result, sputter deposition is not well suited for epitaxial growth of ¯lms.
For polycrystalline ¯lms, the ¯lm grain structure resulting from sputter
deposition typically has many crystallographic orientations without preferred
texture. However, evaporative deposition leads to highly textured
¯lms for which the grain size is typically greater than that of the sputtered
¯lms. Sputter deposition o®ers better control in maintaining stoichiometry
and ¯lm thickness uniformity than evaporative deposition, and has the °exibility
to deposit essentially any crystalline and amorphous materials. These
issues are discussed in more detail in Section 1.8.
1.2.2 Chemical vapor deposition
Chemical vapor deposition is a versatile deposition technique that provides
a means of growing thin ¯lms of elemental and compound semiconductors,
metal alloys and amorphous or crystalline compounds of di®erent stoichiom1.2
Film deposition methods 11

!
Fig. 1.4. Schematic showing the basic features of an open reactor system for chemical
vapor deposition.
etry. The basic principle underlying this method is a chemical reaction between
a volatile compound of the material from which the ¯lm is to be made
with other suitable gases so as to facilitate the atomic deposition of a nonvolatile
solid ¯lm on a substrate, as indicated schematically in Figure 1.4.
The chemical reaction in a CVD process may involve pyrolysis or reduction.
Consider the production of amorphous or polycrystalline Si ¯lms on
Si substrates, where pyrolysis at 650 ◦C leads to the decomposition of silane
gas according to the reaction
SiH4(g) ! Si(s) + 2H2(g).
High-temperature reduction reactions where hydrogen gas is used as a reducing
agent are also employed to produce epitaxial growth of Si ¯lms on
monocrystalline Si substrates at 1200 ◦C according to the reaction
SiCl4(g) + 2H2(g) ! Si(s) + 4HCl(g).
The nature of epitaxy is described in detail later in this chapter.
In CVD, as in PVD, vapor supersaturation a®ects the nucleation rate
of the ¯lm whereas substrate temperature in°uences the rate of ¯lm growth.
These two factors together in°uence the extent of epitaxy, grain size, grain
shape and texture. Low gas supersaturation and high substrate temperatures
promote the growth of single crystal ¯lms on substrates. High gas
supersaturation and low substrate temperatures result in the growth of less
coherent, and possibly amorphous, ¯lms. Low-pressure CVD (LPCVD),
plasma-enhanced CVD (PECVD), laser-enhanced CVD (LECVD) and metalorganic
CVD (MOCVD) are variants of the CVD process used in many
situations to achieve particular objectives.
12 Introduction and Overview
1.2.3 Thermal spray deposition
powder injection
internal external
water
cooling
spray
torch
plasma gases
particles in flight in
molten, semimolten,
or solid state
substrate
air or vacuum chamber
Fig. 1.5. Schematic illustration of the thermal spray process.
The thermal spray process of thin ¯lm fabrication refers broadly to a
range of deposition conditions wherein a stream of molten particles impinges
onto a growth surface. In this process, which is illustrated schematically in
Figure 1.5, a thermal plasma arc or a combustion °ame is used to melt and
accelerate particles of metals, ceramics, polymers or their composites to high
velocities in a directed stream toward the substrate. The sudden deceleration
of a particle upon impact at the growth surface leads to lateral spreading and
rapid solidi¯cation of the particle forming a `splat' in a very short time. The
characteristics of the splat are determined by the size, chemistry, velocity,
degree of melting and angle of impact of the impinging droplets, and by the
temperature, composition and roughness of the substrate surface. Successive
impingement of the droplets leads to the formation of a lamellar structure
in the deposit. The oxidation of particles during thermal spray of metals
also results in pores and contaminants along the splat boundaries. Quench
stresses and thermal mismatch stresses in the deposit are partially relieved
by the formation of microcracks or pores along the inter-splat boundaries
and by plastic yielding or creep of the deposited material. There are several
di®erent types of thermal spray processes, a review of which can be found
in Herman et al. (2000).
Continuous or step-wise gradients in composition through the thickness
of the layer can be achieved by use of multiple nozzles whereby the
1.2 Film deposition methods 13
Fig. 1.6. Scanning electron micrographs showing the microstructures of plasmaspray
coating of NiCrAlY on a 1020 steel substrate. (a) Air plasma spray coated
layer with inter-splat cracks whose origin can be traced to the oxidation of Al in
the coated material during deposition. (b) Vacuum plasma sprayed coating of the
same material without inter-splat microcracks. Reproduced with permission from
Alcala et al. (2001).)
°ow rate of the constituent phases of the deposited composite can be modulated
during spray, as demonstrated by Kesler et al. (1997). Alternatively,
the feed rate of the powders of the di®erent constituent phases can also
be controlled appropriately during thermal spray so as to deposit a graded
layer onto a substrate. Gradients in porosity can be introduced through the
thickness of the deposited layer by manipulating the processing parameters
and deposition conditions.
The plasma spray technique o®ers a straightforward and cost-e®ective
means to spray deposits of metals and ceramics that are tens to hundreds of
micrometers in thickness onto a variety of substrates in applications involving
thermal-barrier or insulator coatings. Typical plasma-spray deposits are
porous, with only 85{90% of theoretical density.
14 Introduction and Overview
For applications requiring higher density coatings with a strong adhesion
to the substrate, low-pressure plasma spray is employed where spraying
is done in an inert-gas container operating at a reduced pressure. Vacuum
plasma spray is another thermal spray process which is used to improve
purity of the deposited material and to reduce porosity and defect content,
albeit at a higher cost than air plasma spray.
Figure 1.6(a) is a representative micrograph of the cross-section of an
air-plasma-sprayed NiCrAlY coating, commonly used as a bondcoat between
a ceramic thermal barrier coating and a nickel-base superalloy substrate in
gas turbine engines, as described in the example in the next subsection. This
coating was deposited onto a 1020 steel substrate. The dark streaks are
the inter-splat boundaries along which microcracks and voids have formed.
The origins of these defects could be traced to the formation of Al2O3 during
deposition (Alcala et al. 2001). On the other hand, vacuum plasma
spray deposition of the same material onto the steel substrate results in the
suppression of such oxidation and the attendant cracking of the inter-splat
boundaries, as shown in Figure 1.6(b). The resulting coating has a more
uniform microstructure with a signi¯cantly reduced pore density.

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